Unable to send emails from Pipex to 02 users

Can anyone help as this is driving me mad. I have recently acquired a couple of chums with 02.co.uk email addies. I simply cannot send them emails from my Pipex Dial email account which works fine for everyone else. I get an error message back which says "5.1.0 - Unknown address error 550-'RCPT TO: [email protected]> Relaying not allowed - please use SMTP AUTH'

[email protected] exists all right, he sent me an email yesterday to prove it. So yes I can receive incoming emails from other 02 users. And yes I have set up SMTP authentication and double checked it. I use a Pipex dial account with OE. I am sending from pop.dial.pipex.com and receiving on smtp.dial.pipex.com. I have set up password confirmation. I am working on port 25.

I know Pipex servers were recently transferred to Tiscali,could that be something to do with it, is Tiscali using some behind the scenes relay which is invisible to me but visible to the 02 servers who are rejecting it. Or is it an 02 problem in which where do I go to sort it out? Tearing my hair out please advise.